The present invention relates generally to improvements in a microwave oven and more particularly to an arrangement for anticipating the occurrence of combustion in the cooking cavity of a microwave oven.
When heating a food load in a microwave oven, if the food load is allowed to heat substantially longer than required for proper cooking, the food itself or the container for the food may become heated to the combustion point, resulting in a fire in the oven cavity. The overcooking of whole potatoes, fats raised to the combustion temperature due to excessive overcooking of meats, and oven shelf hot spots due to imperfections in the pyroceramic shelf when heated over a prolonged period, are three primary sources of such cavity fires. The potato is particularly vulnerable to fire since it dries out non-linearly with the core of the potato getting hot before the outer portions. The heat from the core is not readily dissipated. As a result the core of the potato can be heated to its ignition point relatively quickly.
One commonly used approach to the cavity fire detection problem is to provide a thermal cutout switch on the oven cavity ceiling which cuts off the magnetron when the oven cavity ceiling temperature exceeds a predetermined level. This arrangement works satisfactorily but is relatively costly and has a relatively slow response time. An alternative approach is disclosed in U.S. Pat. No. 4,133,995 to Buck. In the Buck arrangement a humidity sensor and a temperature sensor are employed to monitor the relative humidity and temperature in the cavity. A microprocessor is programmed to detect a fire condition when both the temperature rises and the humidity falls by predetermined amounts within a prescribed time period, on the order of 10-30 seconds, and to de-energize the oven upon detecting such a condition.
The present invention improves over both of the foregoing approaches to fire detection in the microwave oven by providing an arrangement which anticipates fire conditions developing in the cooking cavity before actual ignition employing a single sensor normally used for cooking control which is responsive to the concentration of gases in the circulating air exiting the cooking cavity. In addition to providing an anticipatory warning of an imminent fire condition, this arrangement eliminates the need for the relatively costly thermal cutout switch and also eliminates the need for the "in situ" temperature sensor of the Buck method.